Conventional magnetic write heads used for both disk and tape applications use a single gap to define the shape of written transitions. This geometry is fine for data writing, but is not suitable for writing more complex magnetic patterns such as are necessary for servoing or for tape head module calibration.
Calibration of the position and widths of various read and write elements which comprise a tape head module is necessary to reduce track misregistration errors and therefore increase data density. This calibration may be accomplished by passing the head module over a tape written with a precisely dimensioned pattern. The relative position and absolute widths of the various read elements may be determined by scanning the module laterally across the tape while measuring the signal amplitudes from the various elements. These amplitudes can be used to locate the edges of the read elements. Likewise, write heads can be calibrated by writing a pattern with them and then locating the tracks with previously calibrated read heads. Having servo tracks on the tape can simplify the process of measuring the absolute position of the head module as it scans across the tape.
Servo patterns require a somewhat different structure. They must have some feature which changes across the width of a track. In two-frequency servo schemes, a pattern is written with one frequency on the left side of the track and a different frequency on the right side of the track. As a read head passes over this pattern, its lateral position can be determined by the relative strengths of the two signals. More complex phase-detection schemes are also used. These schemes require that diagonal tracks be written across several tracks. In both cases, these patterns are normally written with multiple passes of a conventional write head. Clearly, the resulting pattern is limited by the write head geometry and the accuracy with which the head can be repeatedly run over adjacent regions of the media.
A timing-based servo scheme has been developed which offers several advantages over more conventional approaches. The idea is to have a servo pattern on the tape which continuously varies from one edge of the servo track to the other. This pattern is then read by a narrow read head which samples the servo information at essentially a single point across the width of the track. The relative timing between successive transitions read by the read head indicates the lateral position of the head. The advantages of this approach are: decreased sensitivity to microtrack profile variations in the read head, improved detection of read errors which can produce erroneous position error signals (PES), and improved linearity of PES signals across the servo track.
A track-following servo control system in a magnetic media storage device is disclosed in application Ser. No. 08/270,207, filed Jun. 30, 1994, and assigned to the same assignee as the present application. The system derives head position information from one or more specially patterned servo tracks. Servo patterns on a tape storage media are generated using a multiple gap servo write head. The gaps of the head contain geometries appropriate to generate desired servo pattern features. The patterned gaps of the head are produced by photolithographically defined plating of permalloy material on a ferrite ring head structure. Pulses of current through the windings of the write head transfer the geometric pattern of the gaps on the head to identical magnetization patterns on the tape. Appropriate timing of the pulses generates the desired pattern sequences. However, this approach requires individual head processing on a previously contoured surface, which is a costly solution.
A number of magnetic write heads are known; see, e.g., U.S. Pat. Nos. 5,296,993; 5,224,260; 5,167,062; 5,123,156; 5,093,980; 5,086,362; 5,067,230; 5,042,140; 5,013,394; 4,939,836; and 4,939,835. However, the magnetic write heads all suffer from one or more disadvantages for the specific application described above. For example, while U.S. Pat. No. 5,296,993 is adequate for its intended purpose, it is capable of only writing flat, and cannot write overlapped patterns. Many of the references are not directed to thin film heads, which are required by the above-mentioned patent application, including U.S. Pat. Nos. 4,939,835; 4,939,836; 5,042,140; and 5,067,230. Others are directed to a mechanically-formed device, including U.S. Pat. Nos. 5,013,394; 5,093,980; 5,123,156; and 5,167,062. The remaining references (U.S. Pat. Nos. 5,086,362 and 5,224,260) employ fabrication processes that are costly.
Thus, a magnetic write head is required that is capable of batch fabrication, employs thin film processing methods, and avoids costly fabrication procedures, yet is useful for the above-described purpose.